1. Technical Field
The present invention relates to a sifting device provided with a sieve made of metal that has excellent classification efficiency, specifically proposes an arrangement of plural holes provided in the sieve, and relates to a sifting device that may enhance efficiency of the sieve and may greatly improve productivity of sifting operations.
2. Related Art
Operation rates of sieves in sifting devices that efficiently sift spherical particles are known to be an important technological factor directly affecting overall industrial productivity. In particular, efficiently sifting spherical particles that are close to perfectly circular, for example, solder balls, is a very important matter in relation to cost, quality and so forth.
Heretofore, shapes of holes in sieves that constitute sifting devices have mostly been circular or square. Moreover, positions of the holes have often been arranged at square grid positions or, more rarely, arranged to be at the points of triangles, and in either case have been arranged uniformly, as what are known as sieve meshes.
When such a sieve mesh is used, the sieve is driven during a sifting operation in a vertical direction or a lateral direction, or alternatively radial directions or the like, and is constantly subjected to oscillations. The object of this oscillation operation is for the particles to touch against the holes of the sieve and then slip downward through the holes as quickly as possible.
However, with vertical oscillations, the particles jump about at the edges of the sieve holes, and may not pass through the holes as hoped. Further, with oscillations to front, rear, left and right, referred to as two-dimensional horizontal oscillations, depending on the speeds and acceleration thereof, there are many opportunities for the particles to pass upward from the holes and therefore the particles may not be efficiently sifted. Further still, when the shapes of the sieve holes are close to the related art squares or perfect circles, that is, when the holes are circumscribed by shortest arcs, the particles stick so as to fill the cavities, and the holes become clogged.
A mechanism by which the particles pass through the holes is that the oscillating particles approach and touch the hole walls, are caught on edge portions of the hole walls, and then fall down. Thus, the longer the lengths of the hole walls through which the particles are acting to pass, the more opportunities there are for the particles to pass through, and the more easily the particles may pass through. Therefore, in a common related art sieve mesh, it is uncertain that there are sufficient chances for particles moving in accordance with lateral direction forces in the plane of the mesh to pass through the holes, and sifting operations are not efficient.
When sifting particles of the order of 20 μm or less, with which the phenomenon of the particles jumping upward occurs, applying positive pressure to the particle side while simultaneously applying negative pressure to the sifted side, such that the sifting operation becomes smoother, is a useful technique. However, a phenomenon then occurs of particles that are temporarily caught in the holes being difficult to remove from the holes, because of the force of the negative pressure, and suchlike. Thus, holes in related art sieve meshes are prone to clogging, and this is not efficient.
In response to this problem, for example, Japanese Patent Application Laid-Open (JP-A) No. 06-170160 proposes a powder separation and removal device in which the shapes of holes in a sieve are made to be long holes, which enhances separation efficiency when sifting powder.
Further, JP-A No. 2006-122826 proposes a sieve for sifting to leave microspheres with a target diameter a, in which the shapes of the holes are rectangular shapes with short sides b whose lengths are 0.9a or less and long sides c whose lengths are greater than a.
Similarly, JP-A No. 11-347491 and JP-A No. 11-47693 propose sieves in which the shapes of the holes are long holes.
However, in the above-mentioned patent references, because the plural long holes formed in each sieve are parallel with one another, when particles are sifted by at least two-dimensional horizontal oscillations, a classification rate is slower in one or other of the oscillation directions.
As indicated above, with related art sieves, numerous investigations have been advanced into making particles slip through the holes and fall down as quickly as possible, preventing clogging of the holes in the sieve meshes, and so forth. However, there is still no definitive means for making sifting operations more efficient.
The present invention is proposed in consideration of the circumstances described above, and the main object is to provide a sieve capable of enhancing sifting efficiency and greatly improving productivity of sifting operations.